Abstract
Recent advances in nanofabrication have enabled the continuing reduction in size of electronic devices. Smaller sizes have led to higher device density at the expense of increased power demand and the resultant heat generation. Electronic devices were damaged by much heat accumulation. The thermal management strategies are thus critically important to continue high performance, reliability and lifetime of devices. Carbon nanotubes possess low density, large aspect ratio and unique thermal properties that make carbon nanotubes be utilized as filler to fabricate nanocomposites. High thermal conductivity nanocomposites based on carbon nanotubes can be developed to assist heat conduction，however, high electrical conductivity of carbon nanotube provides the conductive path causing short circuit of the device at the same time. For solving this problem the technologies of coating inorganic materials on the carbon nanotube is proposed to hinder the electrical conducting channel. There are three chemical processes were proposed for fabricating multi-walled carbon nanotube (MWCNT) coated with inorganic materials. The carboxylic groups were first introduced on the MWCNTs using acid oxidation method for dispersing individual MWCNTs. In order to provide the interfacial interactions between the MWCNTs and the inorganic materials for self-assembly of the nanoinorganic layer, acid oxidized MWCNTs were reacted with a silane coupling agent. Finally, the molecular interaction between the silane functionalized MWCNTs and inorganic materials via sol-gel process was utilized to form the structure of inorganic layer coated on MWCNTs. MWCNTs coated with inorganic materials was analyzed by Raman spectrometer, X-ray photoelectron (XPS), Fourier transform infrared spectrometer (FT-IR) and thermogravimetric analysis (TGA). The morphology of MWCNTs coated with inorganic materials were observed by Transmission electron microscope (TEM).The effect of inorganic materials on the electrical resistivity and thermal conductivity of MWCNTs were investigated. The electrical resistivity of modified MWCNTs was increased 6 orders of magnitude than pristine MWCNTs by coating with inorganic materials .This result exhibits that inorganic layers successfully hinder the electrical conducting channel. The thermal conductivity of MWCNTs was increased from 6.44 W/mK to 8.19 W/mK.The enhancement is 27%. Vinyl ester resin possesses excellent adhesion, mechanical properties and resistance to chemicals, such as acids, alkalies, oxidizing chemicals and salt solutions, etc. However, the thermal conductivity of vinyl ester is too low to be utilized in thermal applications. In this study, MWCNTs coated with inorganic materials were added in vinyl ester to fabricate high thermal conductivtiy and electrical insulation nanocomposite. Inorganic materials coated on MWCNT/vinyl ester nanocomposites were also prepared to investigate the electrical resistivity, the thermal conductivity, glass transition temperature (Tg) and coefficient of thermal expansion (CTE).Results show that inorganic materials coated on MWCNTs can reduce the CTE of nanocomposites and increase Tg, the thermal conductivity of nanocomposites while retain electrical insulation at same time. SEM microphotographs show that the inorganic materials coated on MWCNT / Vinyl ester nanocomposites possess better compatibility and disperity. The volume electrical resistivity decreased 14 orders of magnitude from 7.8 x1015 (ohm*cm) to 4.8 x101 (ohm*cm) with 10phr(parts per hundred parts of resin) of MWCNT. On the other hand, the volume electrical resistivity of Al2O3@SA-MWCNT/vinyl ester nanocomposites decreased slightly from 7.8 x1015 (ohm*cm) to ~5x1012 (ohm*cm).The volume electrical resistivity of nanocomposites decreased only 3 orders of magnitude and maintained electrical insulation . Because the electrical networks of Al2O3@SA-MWCNT in the nanocomposites were hindered by the alumina coating layer. The thermal conductivity of the Al2O3@SA-MWCNT/vinyl ester nanocomposites increased from 0.13 W/mK to 1.12 W/mK when the content of the MWCNTs was increased from 0 to 10 phr. The enhancement is 757% .Tg of the Al2O3@SA- MWCNT/vinyl ester nanocomposites increased from 125. 8oC to 138.9oC when the content of the MWCNT was increased from 0 to 10 phr. The enhancement is 10.4 %.CTE of the Al2O3@SA-MWCNT/vinyl ester nanocomposites decreased from 79. 6 (10-6/K) to 56.2(10-6/K) when the content of MWCNT was increased from 0 to 10 phr.